Heretofore, PTFE formed by an emulsion polymerization method is obtained as an aqueous PTFE emulsion containing PTFE fine particles, by polymerizing tetrafluoroethylene (hereinafter referred to as TFE) in the presence of e.g. an aqueous medium, a polymerization initiator, ammonium perfluorooctanoate (hereinafter referred to as APFO) which is an emulsifier, a stabilizing agent such as paraffin wax (Non-Patent Document 1).
However, it has been pointed out that APFO has an accumulation potential in a body of a small animal, and with respect to an aqueous PTFE emulsion formed by an emulsion polymerization method by using APFO, there has been a concern expressed about an adverse effect to ecosystem. Further, there is the same concern about a PTFE product obtained by using the obtained aqueous PTFE emulsion.
As a PTFE emulsion polymerization by using an emulsifier other than APFO, an emulsion polymerization method to obtain an aqueous PTFE emulsion by using an anionic surfactant which does not contain a fluorine atom, is proposed (for example, Patent Document 1). However, by this method, only an aqueous PTFE emulsion having a relatively low concentration is obtained, and there is a problem such that the obtained PTFE has not enough physical property.
Further, an emulsion polymerization method to obtain an aqueous PTFE emulsion by using a fluorinated emulsifier other than APFO, is proposed (for example, Patent Document 2). However, by this method, the obtained aqueous PTFE emulsion has poor wettability and low permeability when it is attempted to impregnate it into a porous material. Therefore, it has a problem such that when it is applied on a metal plate for coating, it tends to be repelled. Further, this aqueous PTFE emulsion has a low mechanical stability, and there is a problem such that when the emulsion is transferred by a pump, coagulates tend to be formed. The above problems or their solutions are not described in Patent Document 2, and a PTFE product is not described either.
In Examples in Patent Document 3, CF3CF2OCF(CF3)CF2OCF(CF3)COONH4 is disclosed as an emulsifier, but it has been found that this emulsifier has a higher bioaccumulation potential than APFO.
Patent Document 1: JP-B-45-39829
Patent Document 2: JP-B-39-24263
Patent Document 3: JP-A-2003-119204
Non-Patent Document 1: Handbook of Fluororesins P. 28, edited by Takaomi Satokawa, published by Nikkankogyo Shimbun.